1. Field of the Invention
The present invention relates generally to energy conservation and more specifically to a power savings method for rotating pulp and paper machinery, which reduces the rotational speed of a motor, while maintaining a minimum standard of quality.
2. Discussion of the Prior Art
The pulp and paper industry is in desperate need of reducing its energy consumption as the cost of electrical energy is increasing, and the value of the produced paper products is reduced. In most pulp and paper processes, fibers are transported and developed in dilute suspension, in the range of 2-10% of solids, with 90-98% water. For years, optimizations and control strategies have been implemented on pumps and control valves in order to reduce the energy requirements for the transport of the material through the different stages in the process. However, very little has been done for the process equipment that develops the physical properties of the fibrous material. This is primarily due to the fact, that until recently, the cost of electrical energy has been a relatively small part of the production cost. The development of the fibers occurs in rotating machinery such as refiners and pressurized screens. The refiners mechanically work the fibers to make them more flexible, as well as developing the surface area. Pulp stock is passed through a barrier in the pressurized screen to sort out fibers that are not within a certain size and flexibility specification. In addition, any contaminants such as dirt, ink, coarse fibers, stickies etc are transferred to a reject stream for additional processing. Refiners and pressurized screens are both rotating machinery, and a significant portion of the applied power is wasted in terms of a so called no-load. This load is caused by the viscous drag effects as the rotating refiner discs or screen rotor is rotating in the water suspension. Pump theory as well as professional literature show that the no-load is strongly dependent on the diameter of the rotating assembly as well as the rotational speed as illustrated in equation 1.No-load=k*D4.3*w3  (1)
Where D is the diameter and w is the rotational speed. The refiners and pressurized screens are connected to synchronized electrical motors, which means that the rotational speed is fixed in increments of the line frequency. For example a 6 pole motor feed with 60 Hz line current will operate atNs=120*f/p=120*60/6=1200 rpm  (2)
Similarly at 9 poles the synchronous speed will be 800 rpm. With a fixed operational speed, there is always a compromise, since the conditions of the fibers entering the refiners or screens are changing all the time. As a consequence, the rotational speed is always high in order to ensure that the equipment can handle the most extreme fiber conditions, and as a consequence the no-load power use is higher than it needs to be. A graph illustrating no load as a function of rotational speed for two typical refiner diameters is shown in FIG. 1.
U.S. Pat. No. 4,661,911 to Ellery, Sr. discloses an adaptive constant refiner intensity control. The Ellery, Sr. patent includes a method and an apparatus for maintaining a constant refining intensity under varying tonnage rate and applied power conditions to a slurry of paper stock being passed through a disk type refiner.
U.S. Pat. No. 5,450,777 to Molnar et al. discloses a method and apparatus for processing chopped fibers from continuous tows. The Molnar et al. patent includes an apparatus for processing chopped fibers. The apparatus includes sensors for monitoring the movement of each of the tows to the pump and cutter mechanism, and a controller, which adjusts the feed rate of the tows.
U.S. Pat. No. 6,336,602 to Miles discloses a low speed low intensity chip refining (in a steam phase, not in water). The Miles patent includes mechanical refining of wood chips. Wood pulp quality is improved by employing low refining intensity at least in a final refining stage. The refining is carried out at rotational speeds that are lower than those conventionally employed. However, it should be noted that this work relates to the net energy inputted into the fibers and not to no-load energy.
Accordingly, there is a clearly felt need in the art for a power savings method for rotating pulp and paper machinery, which reduces the rotational speed of a motor and proportionally reduces the amount of energy consumption, while maintaining a minimum standard of pulp quality.